Wear-resistant and low-friction coatings and articles coated therewith

ABSTRACT

A composition for a wear-resistant and low-friction coating is presented. The coating composition includes a hard ceramic phase, a metallic binder phase and a lubricant phase. The lubricant phase includes a multi-component oxide. An article having a wear-resistant and low-friction coating and a method of making such a coating are also described.

BACKGROUND

This invention generally relates to coatings for articles. Particularly,the invention relates to protective coatings, which provide wearresistance and low-friction characteristics to articles exposed to hightemperatures. Examples of such articles include turbomachines, such asturbine engines.

Metal components are used in a wide variety of industrial applications,under a variety of operating conditions. In many cases, the componentsare provided with coatings, which impart various characteristics, suchas corrosion resistance, heat resistance, oxidation resistance, and wearresistance. As one example, the various components of turbine enginesare often coated with thermal barrier coatings, to effectively increasethe temperature at which they can operate. Other examples of articlesthat require some sort of protective coating include pistons used ininternal combustion engines and other types of machine.

Wear-resistant coatings (often referred to as “wear coatings”) arefrequently used on turbine engine components, such as nozzle wear padsand dovetail interlocks. The coatings provide protection in areas wherecomponents may rub against each other, since the rubbing—especially highfrequency rubbing—can damage the part. A specific type of wear isreferred to as “fretting”. Fretting can often result from very smallmovements or vibrations at the junction between mating components, e.g.,in the compressor and/or fan section of gas turbine engines. Forexample, fretting can occur in regions where fan or compressor bladesare joined to a rotor or rotating disc. This type of wear cannecessitate premature repair or replacement of one or more of theaffected components. Various alloys, such as those based on nickel orcobalt, are susceptible to fretting and other modes of wear. Manytitanium alloys have especially poor fretting characteristics. Alongwith fretting resistance, other characteristics for the coating are alsorequired. These include anti-scuffing properties (for example, in thecase of piston rings and cylinder liners), as well as anti-frictionproperties.

A variety of coating systems have been used to impart wear resistance tosubstrates. Examples include those based on chromium; chromium carbide;cobalt-molybdenum-chromium-silicon, or copper-nickel-indium. Thecoatings can be applied by a variety of techniques, such as plating orthermal spraying.

While hard chromium coatings have been of great use in variousapplications, they exhibit some drawbacks. For example, the integrity ofthese coatings is challenged by the high temperatures and pressures towhich they are often subjected, in both aerospace and automotive engineapplications. Furthermore, chromium plating can be a very time-consumingprocess.

Moreover, the toxicity of some of the chromate compounds used as thechromium source is another drawback to the plating processes. Inparticular, hexavalent chromium is considered to be a carcinogen. Whencompositions containing (or releasing) this form of chromium are used,special handling procedures have to be very closely followed, in orderto satisfy health and safety regulations. The special handlingprocedures can often result in increased costs and decreasedproductivity.

In many applications, chromium plating processes have been replaced byspraying techniques, as mentioned above. As an illustration, thermalspray techniques have been employed to deposit coatings based ontungsten carbide (WC), or chromium carbide (for example, Cr₃C₂). Whilethe resulting coatings are suitable for many purposes, they havelimitations as well, e.g., in their thermal properties.

Thermally sprayed cermet coatings have also become fairly popular forproviding wear resistance in certain situations. Examples of thesecoatings include tungsten carbide-cobalt (WC—Co), tungstencarbide-cobalt-chromium (WC—Co—Cr), and chromium carbide/nickel chromium(for example, Cr₃C₂—NiCr). As another example, U.S. Pat. No. 6,887,585(Herbst-Dederichs) describes wear-resistant coatings based on a metallicphase, such as nickel or iron alloys, along with a ceramic phase, suchas alumina, chromic oxide (Cr₂O₃), or titanium carbide (TiC). Thecoatings may also include a solid lubricant material to reduce friction.Examples of the lubricants include materials such as graphite, hexagonalboron nitride, and polytetrafluoroethylene.

While many of the cermet coatings are suitable for certain end uses,they also exhibit deficiencies. For example, the hardness of the WC— andCr₃C₂-based coatings may be insufficient for other end uses, asmentioned above. Moreover, WC—Co coatings are usually restricted totemperatures below about 500 degrees Celsius, in oxidizing environments.This limitation is often seen in a thermal spray process such as HVOF orAPS, and is due in part to carbide degradation during deposition.Carbide degradation can occur when a WC material is oxidized, and formsbrittle sub-carbides. Coatings like those based on Cr₃C₂—NiCr may havesatisfactory wear properties at higher temperatures, e.g., about500-900.degree. C. However, they may not have adequate low-frictionproperties and, moreover, it is difficult to control the microstructureof such coatings during thermal spraying.

Thus, there is a need to provide an improved protective coating forcompressor components, particularly to those components that are exposedto wear and friction. It is desirable that the protective coatingprovides the necessary amount of wear-resistance (e.g., anti-frettingcapabilities) desired for high temperature applications, and alsoproviding good low-friction properties.

BRIEF DESCRIPTION

One embodiment is a coating composition including a hard ceramic phase,a metallic binder phase and a lubricant phase. The lubricant phaseincludes a multi-component oxide.

Another embodiment is an article. The article includes a metallicsubstrate and a wear-resistant and low-friction coating disposed on thesubstrate. The coating composition includes a hard ceramic phase, ametallic binder phase and a lubricant phase, wherein the lubricant phaseincludes a multi-component oxide.

Yet another embodiment is a method of making a composition for awear-resistant and low-friction coating. A hard ceramic phase and ametallic binder phase are milled together to make a mixture and then, alubricant phase is dispersed in the mixture. The lubricant phaseincludes a multi-component oxide.

DRAWINGS

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings,wherein:

FIG. 1 shows scanning electron micrographs of Sample 1 before and afterfriction test.

FIG. 2 shows scanning electron micrographs of Sample 2 before and afterfriction test.

DETAILED DESCRIPTION

Embodiments of the present invention include in part a wear-resistantand low-friction coating composition for substrates, such as metalsubstrates. The coatings may be particularly useful for high temperatureapplications, and may provide other benefits, including desirable levelsof hardness, corrosion resistance, heat resistance, and oxidationresistance. Moreover, the coatings may be applied by a variety ofthermal spray processes. The coating is particularly well suited forprotecting metallic components, such as industrial gas turbinecompressor components that are often formed of iron-based alloys. Thesecomponents are generally formed of martensitic/ferritic stainless steelsand subjected to degradation (fretting, etc.). While the invention willbe described in reference to compressor components formed of a stainlesssteel, it should be understood that the teachings of this invention willapply to other components that are formed of a variety of metals,including, for example, iron-based alloys, superalloys (such asnickel-based and cobalt-based superalloys) and titanium-based alloys;such components may also benefit from improved wear-resistance andreduced friction.

Approximating language, as used herein throughout the specification andclaims, may be applied to modify any quantitative representation thatcould permissibly vary without resulting in a change in the basicfunction to which it is related. Accordingly, a value modified by a termor terms, such as “about,” is not limited to the precise valuespecified. In some instances, the approximating language may correspondto the precision of an instrument for measuring the value.

In the following specification and the claims that follow, the singularforms “a”, “an” and “the” include plural referents unless the contextclearly dictates otherwise.

As used herein, the terms “may” and “may be” indicate a possibility ofan occurrence within a set of circumstances; a possession of a specifiedproperty, characteristic or function; and/or qualify another verb byexpressing one or more of an ability, capability, or possibilityassociated with the qualified verb. Accordingly, usage of “may” and “maybe” indicates that a modified term is apparently appropriate, capable,or suitable for an indicated capacity, function, or usage, while takinginto account that in some circumstances the modified term may sometimesnot be appropriate, capable, or suitable. For example, in somecircumstances, an event or capacity can be expected, while in othercircumstances the event or capacity cannot occur—this distinction iscaptured by the terms “may” and “may be”.

According to one embodiment of the invention, a coating compositionincludes a hard ceramic phase, a metallic binder phase, and a lubricantphase. The metallic binder phase functions as a binder for the hardceramic phase and the lubricant phase in the coating composition. Choiceof the constituents for a particular metallic binder depends on avariety of factors. One factor relates to the type of hard ceramicparticles employed, and the ability of the metallic binder to adequately“wet” the hard ceramic particles. Another factor involves performanceparameters for particular end uses for the coating, for example, interms of characteristics such as corrosion resistance, heat resistance,oxidation resistance, and wear resistance. Another factor relates to thepotential interaction of metallic binder with the other constituents,for example, the potential formation of undesirable compounds or phasesat elevated temperatures.

Usually, the metallic binder phase (also referred to as metallic phase)is based on at least one of nickel, cobalt, iron, copper, and silver. Insome embodiments, nickel is a constituent for the metallic phase. Asfurther described below, combinations of nickel and chromium are alsoused in some embodiments.

The metallic phase very often includes a variety of other elements,depending on many of the factors discussed previously. Non-limitingexamples are refractory elements such as tantalum, niobium, zirconium,and, molybdenum; as well as titanium, chromium, silicon, boron, andvanadium. Many combinations of these elements may also be employed, andthe selection of any element or combination thereof will depend on manyof the criteria noted above. As an example, niobium can be included toprovide ductility and strength, while chromium, zirconium, and siliconmay be added to enhance oxidation resistance. In some instances, boronand silicon are also added for melting point suppression, while chromium(mentioned above) and molybdenum are often added for corrosionresistance.

A non-limiting illustration of the amount of components in a coatingcomposition can be provided, for some embodiments. For certaincompositions in which the coating comprises at least about 50% by weightnickel, typical ranges for other constituents (if present) may be asfollows (based on total coating composition, in weight):

Ta: about 0.5% by weight to about 1.5% by weight;

Ti: about 0.5% by weight to about 2% by weight;

Nb: about 0.5% by weight to about 2% by weight;

Cr: about 2% by weight to about 50% by weight;

Zr: about 0.5% by weight to about 1% by weight;

Si: about 0.5% by weight to about 4.5% by weight;

B: about 0.5% by weight to about 3.5% by weight; and

Mo: about 0.5% by weight to about 18% by weight.

In some embodiments, the metallic phase itself comprises nickel andchromium. For example, the matrix could include about 70% to about 90%nickel and about 5% to about 25% chromium, based on the total weight ofthe metallic phase, with the balance comprising one or more of theelements listed above. In certain embodiments, the metallic phasecomposition comprises nickel, chromium and molybdenum. In that instance,the metallic phase may include about 50% to about 80% nickel; about 5%to about 20% chromium, and about 10% to about 30% molybdenum, with thebalance comprising the other elements described previously.

Various other combinations of metals for the metallic phase are alsoused in some embodiments. Non-limiting examples include cobalt andchromium; iron and chromium; iron and manganese; and iron and cobalt.Those skilled in the art will be able to select the most appropriatemetallic phase composition for a particular situation, based in part onthe teachings herein. Usually, the metallic phase is present at a levelin the range of about 1% by volume to about 50% by volume, based on thetotal volume of the coating composition. In some specific embodiments,the metallic phase is present at a level in the range of about 5% byvolume to about 20% by volume. The metallic phase often have particlesof particle size in the range from about 0.5 micron to about 5 microns.

As mentioned above, the coating composition also includes at least onehard ceramic phase (i.e., a “primary ceramic phase”), which can providethe required amount of wear resistance and load-bearing characteristicsfor a given application. As used herein, the term “ceramic” may includea variety of hard-phase materials, for example, carbides, borides, oroxides of a metal selected from the group consisting of chromium,tantalum, molybdenum, vanadium, zirconium, and niobium. The ceramicphase can be formed from one or more constituents including those addedinitially for preparing coating composition as well as those formedin-situ when the coating performs at high temperatures. For example,carbides or borides in the coating composition may decompose into oxideswhile operating coated components at high temperatures. It will beunderstood the term “hard” as used herein refers to the hardness of thephase relative to the other constituents of the coating; that is, theceramic constituent with the highest level of hardness is considered the“hard” ceramic phase.

Carbide-containing ceramic materials are used in some instances. Inexamplary embodiments, the hard ceramic phase (also referred to hereinas simply “ceramic phase”) comprises chromium carbide. The chromiumcarbide is typically one or more materials selected from the groupconsisting of Cr₃C₂, Cr₇C₃, and Cr₂₃C₆. Other examples of the ceramicphase include various borides. As used herein, “boride” is meant toinclude, without limitation, diborides and other boride species, unlessotherwise specified. Non-limiting examples of boride include titaniumdiboride, zirconium diboride tantalum boride, and tungsten boride.General examples of the compounds include the transition metaldiborides.

The selection of particular ceramic phase constituents depends in parton the factors described previously. For example, coating compositionsthat will protect turbine parts subjected to a high degree of frettingoften include ceramic phase constituents because they provide arelatively high degree of abrasion resistance and wear resistance.

In specific embodiments, the coating composition further comprises asecondary ceramic phase. The secondary phase can function to increasethe overall toughness of the composition. The phase usually comprises atleast one material selected from the group consisting of siliconcarbide, various metal carbides (e.g., boron carbide, titanium carbide,and tungsten carbide); various metal oxides (e.g., titanium dioxide andalumina); titanium nitride, and diamond. Further non-limiting examplesof suitable materials include alumina, yttrium oxide, yttria-stabilizedzirconia, hafnium oxide, silicon oxide (silicon dioxide), and mullite.Combinations of any of these materials are also possible. In some cases,the secondary phase includes alumina, titanium nitride, diamond dust, orvarious combinations thereof. The secondary ceramic phase, if included,is usually present at less than about 30 volume % of the entire ceramicphase.

The particles of the primary ceramic phase usually have particle size ofat least about 0.2 micron, and up to about 5 microns. In some specificembodiments, the particle size is in the range of about 0.2 micron toabout 3 microns, and more specifically, in the range of about 1 micronto about 1.5 microns. Moreover, in some preferred embodiments, thesecondary ceramic phase has a finer particle size than that of theprimary phase. The particle size for the secondary ceramic phase, insome embodiments, is less than about 1 micron, and in particularembodiments, less than about 100 nanometers. In certain embodiments, thesecondary phase has particle size in the range from about 10 nanometersto about 100 nanometers.

The amount of the hard ceramic phase varies considerably, depending onmany of the factors described herein, including, for example, theparticular type of ceramics being used, as well as the desired hardnessfor the coating composition. In general, the ceramic phase is present ata level in the range from about 20% by volume to about 90% by volume,based on the volume of the entire coating composition. In some specificembodiments, the amount of the ceramic phase is in the range from about40% by volume to about 80% by volume. In some embodiments, the amount isin the range from about 50% by volume to about 70% by volume.

As noted previously, the coating composition includes a lubricant phase(also referred to as lubricant). The presence of the lubricant provideslubricity to the coating, to decrease the friction between two surfacesrubbing against each other. The particular lubricant selected willdepend on various factors. Wear-resistance, friction-coefficient andoperating temperature are in part key considerations. Compatibility ofthe lubricant with the materials which are used for the metallic phaseand ceramic phase is also an important consideration.

As used herein, the term “lubricant phase” or “lubricity of a phase”refers to describe the ability of a compound (lubricant) to reducefriction between two or more sliding parts in a machine or mechanism.Another parameter to determine lubricity of a compound may be its shearrheology.

In some embodiments, the lubricant phase is formed of a multi-componentoxide. As used herein, the term “multi-component oxide” refers to acombination of at least a pair of oxides. For example, themulti-component oxide may be a binary oxide (two component oxides), aternary oxide (three component oxides) or a quaternary oxide (fourcomponent oxides). The constituent oxides in the combination (mixture)may or may not chemically react. Chemical reaction among constituentoxides may depend on various parameters such as types of constituentoxides, method of preparation of a multi-component oxide, environmentalconditions etc. In one embodiment, the constituent oxides do not reactand remain as individual oxides in the mixture. However, chemicalreaction may occur at the operating temperature. In another embodiment,the constituent oxides are reacted with one-another (either before orafter introducing them into the coating composition) and form a complexcompound, and it is this complex compound that serves as the lubricantphase.

In general, the higher the ionic potential, the lower the frictioncoefficient of an oxide. “Ionic potential” is a ratio of electric chargeto the radius of an ion, and thus a measure of density of charge of theion. Ionic potential gives a sense of how strongly or weakly the ion iselectrostatically attracted to ions of opposite charge or repelled byions of like charge. The oxides with higher ionic potential appear toshear more easily and thus exhibit lower friction at high temperatures.Moreover, as the difference in ionic potentials of the oxideconstituents of a multi-component oxide increases, the ability of oxidesto form a low melting point or readily shearable compound improves andhence the multi-component oxide exhibits low hardness and shearstrength. In other words, the greater the difference in ionic potentialsof the component oxide constituents of the lubricant phase, lower is thefriction at elevated temperatures. For example, a binary oxide NiO—B₂O₃having large ionic potentials difference (˜9 Å⁻¹) exhibits low frictioncoefficient (˜0.2) at 600 degrees Celsius. Furthermore, the ability ofan oxide to dissolve in or react with other oxide or to form complexcompounds increase with the difference in their ionic potentials. Themechanism of such lubricious oxides is described in an article entitled“A crystal-chemical approach to lubrication by solid oxides” by AliErdemir, Tribology Letters 8 (2000), 97-102.

In some embodiments, at least one oxide of the multi-component oxidelubricant phase has an ionic potential greater than about 4 Å⁻¹, and insome certain embodiments, greater than about 5 Å⁻¹. In certainembodiments, the lubricant phase contains a multi-component oxide havingat least one metal oxide constituent selected from the group consistingof nickel oxide, alumina, titanium oxide, tantalum oxide, zinc oxide,molybdenum oxide and magnesium oxide.

In certain instances, a binary oxide is used as lubricant in the coatingcomposition. The binary oxide contains a pair of component oxides. Asdiscussed above, choice of the oxide pair depends on their ionicpotentials. As the difference in ionic potential increases, thelubricity of the binary oxide (or oxide pair) increases, especially atelevated temperatures. Examples of suitable oxide pairs include, but arenot limited to, NiO—B₂O₃, NiO—TiO₂, NiO—Ta₂O₅ and MgO—SiO₂. In certainembodiments, the binary oxide includes NiO—B₂O₃. In other certainembodiments, the binary oxide includes NiO—TiO₂.

Lubricious properties may further depend on the amount of constituentoxides relative to each other present in a binary oxide. In someembodiments, the constituent oxides of the multi-component oxide arepresent in a ratio (by weight) varying from about 1:1 to about 1:10. Incertain embodiments, the constituent oxides are present in a ratio (byweight) varying from about 1:1 to about 1:5, and in some specificinstances, from about 1:1 to about 4:1.

The desired particle size of the lubricant depends on the particularmaterial being used. A particle size, which is too small, may decreasethe beneficial effect of the lubricant in reducing friction. Conversely,if the particle size of the lubricant is too large, tribological andmechanical properties may suffer. For example, the mechanical strengthas well as the wear resistance of the coating may decrease. Generally,the lubricant particles have a size that permits them to be situatedwithin the spacing that separates the hard particles in the metallicbinder. In some embodiments, the lubricant phase contains submicron sizeparticles of the multi-component oxide. As used herein, the term“submicron” refers to particle dimension in a range from about 100nanometers to about 2 microns. In certain embodiments, the oxideparticles have particle dimension in a range from about 50 nanometers toabout 1 micron.

The amount of lubricant in the coating composition may depend on many ofthe factors mentioned previously. As an example, an excessive amount oflubricant (or lubricant particles which are too large) may decrease themechanical strength of the coating. In some instances, the lubricantphase is present at a level in the range of about 1% by volume to about30% by volume of the coating composition. In certain instances, therange for lubricant is about 5% by volume to about 20% by volume.

The coating composition may be designed to provide corrosion resistancealong with wear resistance and lubrication. This can be achieved bytailoring the amount of the coating constituents to a specificenvironment known to exist in a given application. In some instances,the amount of the metallic binder phase and the in-situ generated phasesmay be tailored to be corrosion resistant to a given environment.

In some embodiments, the lubricant component is incorporated into thecoating composition in solid powdered form or slurry form. A number ofmethods may be used to form oxide powders or a slurry having particlesof above mentioned particle size. Examples of some methods are theprecipitate method and the solid-state method. In some instances, themulti-component oxide powder is synthesized by a chemical route.

The coating composition advantageously provides high wear resistance andgood lubrication. Finely dispersed oxide particles provide a lowfriction and low shear strength phase and in-situ oxide formationprovides high wear resistance to sliding surfaces under heavy loads. Thein-situ generated oxide, for example chromium oxide inNiCr—Cr₃C₂—NiO—B₂O₃ composition, contributes to good wear resistance forthe coating. In certain instances, the atomic ratio of chromium tooxygen in the coating is about 1 (desirable for good wear resistance).Furthermore, a smooth smeared-like layer is formed during wear, as thesurfaces slide across each other under heavy loads. The layer is capableof providing low friction and low wear rate. The multi-component oxidepresent in the coating enables good lubrication even when the topsurface of the coating is removed by wear.

Some embodiments provide a method of making the coating compositionsdescribed above. Mechanical milling procedures can be used to prepare amixture of a hard ceramic phase and a metallic binder phase. In thoseinstances, a high-energy mill may be used to carry out the millingprocess. The next step involves dispersion of the lubricant phase in theabove mixture by adding and further milling the mixture to form thecomposition. Some other examples of suitable techniques includespray-drying, self-propagating, and high-temperature synthesis (SHS).Another suitable technique involves sintering of the raw materialpowders, followed by crushing of the resulting pellets. The preparationtechniques may involve multiple steps, and combinations of varioustechniques.

An article, according to one embodiment of the invention, includes ametallic substrate that may be a component of an industrial gas turbine.Specific, non-limiting examples of the turbine components includebuckets, nozzles, blades, rotors, vanes, stators, shrouds, combustors,and blisks. Non-turbine applications are also possible. Examples furtherinclude components of other articles used under conditions of hightemperature and/or high-wear environments. These components (e.g., thesubstrate) are typically formed of a metal or a metal alloy, such asstainless steel. Other suitable materials for the substrate includenickel, cobalt, titanium, and their respective alloys. The componentsmay be coated or partially coated with the coating composition forprotecting surfaces from the ambient environment.

The thickness of the coating may depend on many of the other factorsdiscussed previously, for example, composition of the coating andarticle, the end use of the article, and the like. In some embodiments,the coating will have a thickness of about 50 microns to about 500microns. In some specific embodiments, the thickness will be in therange of about 100 microns to about 200 microns.

As mentioned previously, the coatings described herein are particularlyuseful for deposition on a metal alloy, which includes a contact surfacethat is shaped or positioned to cooperate with the contact surface of anabutting member. In such an instance, the coating (which could also beapplied to the abutting member) substantially prevents fretting wearbetween the contact surfaces. It is believed that the coatings aresuitable for supporting high-contact stresses between such surfaces,e.g., stresses greater about 30,000 psi. Moreover, the coatings may beuseful when employed under oxidizing conditions at elevatedtemperatures, for example, above about 500 degree Celsius, and in someinstances, even above about 600 degrees Celsius.

The coating composition can be applied to the substrate by a variety ofdifferent techniques. Selection of a particular technique will depend onvarious factors, such as the type and composition of the coating powder,feedstock particle size, and the end use contemplated for the part. Inone embodiment, a spray technique is used to deposit the coating.Non-limiting examples include plasma deposition (e.g., ion plasmadeposition, vacuum plasma spraying (VPS), low pressure plasma spray(LPPS), and plasma-enhanced chemical-vapor deposition (PECVD)); HVOFtechniques; high-velocity air-fuel (HVAF) techniques; PVD, electron beamphysical vapor deposition (EBPVD), CVD, APS, cold spraying, and laserablation.

Thermal spray techniques are of special interest for some embodiments.Examples listed above include VPS, LPPS, HVOF, HVAF, APS, andcold-spraying. In many instances, HVOF or HVAF is the preferredtechnique. Those skilled in the art are familiar with the operatingdetails and considerations for each of these techniques. Moreover,various combinations of any of these deposition techniques could beemployed. It should also be noted that in some preferred embodiments,thermally sprayed coatings are polished after being deposited. Thesesteps provide a degree of surface roughness, which enhanceswear-resistance, and decreases friction characteristics.

EXAMPLES

The examples that follow are merely illustrative, and should not beconstrued to be any sort of limitation on the scope of the claimedinvention.

Preparation of binary oxide

Example 1 Solid State Method of Preparing of NiO—B₂O₃

2.772 grams of NiO powder and 2.585 grams of B₂O₃ powder were milledtogether using a rack mill for about 3-6 hours. This powder was thenextracted and heat treated at about 400 degrees Celsius for about 1 hourfollowed by heat treatment at 900 C for about 1 hour.

Example 2 Alternate Solid State Method of Preparing NiO-B₂O₃

2.772 grams B₂O₃ or 4.924 grams H₃BO₃ was dissolved in water and thenmixed along with 4.28 grams of NiO or 5.313 grams nickel hydroxideNi(OH)₂ powder. After milling for about 3-6 hours, the water wasevaporated and the extracted powder was subjected to heat treatment atabout 400 degrees Celsius for about 1 h and then about 900 degreesCelsius for about 1 hour.

Example 3 Formation of a Coating Composition (Sample 1)

8.986 grams of NiCr powder and 60.656 grams of Cr₃C₂ powder were milledin a high-energy mill using a powder-to-media ratio of about 1:15 forabout 10 hours. This mixture was further mixed with 5.357 grams ofNiO—B₂O₃ powder, synthesized using the solid-state method described inexample 1. This mixing was carried out in a rack mill for about 1 hour.The powder had particles of particle size less than about 10 micronswith about 90 percent particles of particle size about 6 microns.

Example 4 Formation of a Coating Composition (Sample 2)

8.986 grams of NiCr powder, 60.656 grams of Cr₃C₂ powder and 5.357 gramsof NiO—B₂O₃ powder (prepared in-house by method described in example 1)were loaded into a polypropylene bottle containing milling media andisopropanol. Milling media to powder ratio was kept about 1:15.Isopropanol was added such a way that the solid loading in the resultantslurry was in a range of about 60 wt % to 80 wt %. Milling was carriedout for about 10-12 hours and a powder was recovered by solventevaporation. The powder had particles of particle size less than about10 microns with about 90 percent particles of particle size about 6microns.

Example 5 Formation of Test Samples

Two powders prepared in example 3 (Sample 1) and example 4 (Sample 2)were pressed in a uniaxial press to form pellets of 25 millimeters indiameter. Sample 1 and Sample 2 were sintered in about 4% hydrogenbalance argon atmosphere for about 1 hour.

Each sample was subjected to a “pin on disk” friction measurement test.Measurements were taken using a 6 mm diameter tungsten carbide ballunder contact pressure of about 1.18 GPa, temperature of about 800degrees Celsius and speed of about 5 cm/sec. A transducer measured thewear rate and the coefficient of friction. Substantially no wear wasobserved during 1-hour friction test and 150 meters of sliding.Furthermore, friction coefficients of sample 1 and sample 2 were lessthan about 0.25, which is comparatively less than the frictioncoefficient (−0.3) for wear coatings having boron nitride as lubricantphase. Furthermore, FIG. 1 and FIG. 2 illustrate microstructures ofSample 1 and Sample 2 before and after the friction measurements.Microstructures of Sample 1 and Sample 2 after the friction measurementsshow formation of a smooth surface having low friction and low wearrate.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

1. A coating composition, comprising: a hard ceramic phase, a metallicbinder phase; and a lubricant phase comprising a multi-component oxide.2. The coating composition of claim 1, wherein the metallic binder phaseis present at a level in the range from about 1% by volume to about 50%by volume of the total volume of the composition.
 3. The coatingcomposition of claim 1, wherein the hard particle phase is present at alevel in the range from about 20% by volume to about 90% by volume ofthe total volume of the composition.
 4. The coating composition of claim1, wherein the lubricant phase is present at a level in the range fromabout 1% by volume to about 20% by volume of the total volume of thecomposition.
 5. The coating composition of claim 1, wherein the metallicbinder phase comprises at least one metal selected from the groupconsisting of nickel, cobalt, iron, copper, silver and combinationsthereof.
 6. The coating composition of claim 1, wherein the metallicbinder phase further comprises at least one metal selected from thegroup consisting of tantalum, titanium, chromium, niobium, zirconium,molybdenum, silicon, boron, and vanadium.
 7. The coating composition ofclaim 1, wherein the metallic binder phase comprises nickel andchromium.
 8. The coating composition of claim 7, wherein the metallicbinder phase comprises at least about 50% by weight nickel, based on thetotal weight of the metallic phase.
 9. The coating composition of claim1, wherein the hard ceramic phase comprises a carbide, a boride, or anoxide of at least one element selected from the group consisting oftungsten, aluminum, chromium, tantalum, modyblednum, vanadium,zirconium, niobium or a combination thereof.
 10. The coating compositionof claim 9, wherein the hard ceramic phase comprises chromium carbide.11. The coating composition of claim 9, wherein the boride is selectedfrom the group consisting of titanium diboride, zirconium diboride,tantalum boride, tungsten boride, and a combination thereof.
 12. Thecoating composition of claim 1, wherein the hard ceramic phase comprisesparticles having a particle size in the range from about 0.1 micron toabout 100 microns.
 13. The coating composition of claim 1, wherein themetallic binder phase comprises particles having a particle size in therange from about 0.1 micron to about 5 microns.
 14. The coatingcomposition of claim 1, wherein the multi-component oxide comprises atleast one oxide having ionic potential greater than about 4 k′.
 15. Thecoating composition of claim 14, wherein the multi-component oxidecomprises at least one oxide having ionic potential greater than about 5k′.
 16. The coating composition of claim 1, wherein the multi-componentoxide is a binary oxide, a ternary oxide or a tetranary oxide.
 17. Thecoating composition of claim 16, wherein the multi-component oxidecomprises at least a metal oxide selected from the group consisting ofnickel oxide, alumina, titanium oxide, tantalum oxide, zinc oxide,molybdenum oxide and magnesium oxide.
 18. The coating composition ofclaim 1, wherein the multi-component oxide is a binary oxide.
 19. Thecoating composition of claim 18, wherein the binary oxide is selectedfrom the group consisting of NiO—B₂O₃, NiO—TiO₂, NiO—Ta₂O₅ and MgO—SiO₂.20. The coating composition of claim 18, wherein the binary oxidecomprises constituent oxides present in a ratio (by weight) varying fromabout 1:1 to about 1:10.
 21. The coating composition of claim 20,wherein the binary oxide comprises constituent oxides present in a ratio(by weight) varying from about 1:1 to about 1:5.
 22. The coatingcomposition of claim 1, wherein the lubricant phase comprises particleshaving a particle size in the range from about 0.05 microns to about 20microns.
 23. The coating composition of claim 22, wherein the lubricantphase comprises particles having a particle size in the range from about0.1 microns to about 10 microns.
 24. An article comprising: a metallicsubstrate; and a wear-resistant and low-friction coating disposed on thesubstrate, wherein the coating composition comprises: a hard ceramicphase, a metallic binder phase; and a lubricant phase comprising amulti-component oxide.
 25. The article of claim 24, wherein the metallicsubstrate comprises a component of turbine engine.
 26. The article ofclaim 24, wherein the metallic substrate comprises a superalloy based onnickel, cobalt, iron, aluminum, or titanium.
 27. The article of claim24, wherein the multi-component oxide is a binary oxide.
 28. The articleof claim 27, wherein the binary oxide is selected from the groupconsisting of NiO—B₂O₃, NiO—TiO₂, NiO—Ta₂O₅ and MgO—SiO₂.
 29. Thearticle of claim 27, wherein the binary oxide comprises constituentoxides present in a ratio (by weight) varying from about 1:1 to about10:1.
 30. The article of claim 29, wherein the binary oxide comprisesconstituent oxides present in a ratio (by weight) varying from about 1:1to about 1:5.
 31. A method of making a composition for a wear-resistantand low-friction coating, comprising the step of: milling a hard ceramicphase and a metallic binder phase to make a mixture; and dispersing alubricant phase in the mixture, wherein the lubricant phase comprises amulti-component oxide.
 32. The method of claim 31, wherein milling iscarried out in a high energy mill.
 33. The method of claim 31, whereindispersing comprises adding and milling the lubricant phase with themixture.